Inspection device, image forming apparatus, and inspection method

1. An inspection device comprising: an image acquirer that acquires an inspection target image, the inspection target image being a read image obtained by reading an inspection side of a recording material on which an output image is formed; an edge extractor that extracts an edge from each of the inspection target image and a reference image to be used in inspecting the output image; a defect candidate region extractor that extracts a defect candidate region having a possibility of a defect by comparing the inspection target image with the reference image; an edge direction calculator that calculates a direction of the edge in the inspection target image and a direction of the edge in the reference image; and a defect determiner that determines whether the defect candidate region is a defect, on a basis of the direction of the edge in the inspection target image and the direction of the edge in the reference image at a position corresponding to the defect candidate region.

2. The inspection device according to claim 1 , wherein the defect determiner compares the direction of the edge in the inspection target image with the direction of the edge in the reference image, and, when a difference between the directions of the edges is equal to or larger than a first threshold, determines the defect candidate region to be a defect.

3. The inspection device according to claim 2 , further comprising an edge gradient calculator that calculates a magnitude of a gradient of the edge in the inspection target image and a magnitude of a gradient of the edge in the reference image at the position corresponding to the defect candidate region, wherein, when a difference between the magnitudes of the gradients of both edges is equal to or larger than a second threshold, the defect determiner determines the defect candidate region to be a defect even when the difference between the directions of the edges in the inspection target image and the reference image is smaller than the first threshold.

4. The inspection device according to claim 1 , further comprising an edge distance calculator that calculates a distance from the position corresponding to the defect candidate region to the nearest edge in the reference image, wherein, when the distance is equal to or greater than a third threshold, the defect determiner determines the defect candidate region to be a defect.

5. An image forming apparatus comprising: an image former that forms an output image on a recording material; an image acquirer that acquires an inspection target image, the inspection target image being a read image obtained by reading an inspection side of the recording material on which the output image is formed; an edge extractor that extracts an edge from each of the inspection target image and a reference image to be used in inspecting the output image; a defect candidate region extractor that extracts a defect candidate region having a possibility of a defect by comparing the inspection target image with the reference image; an edge direction calculator that calculates a direction of the edge in the inspection target image and a direction of the edge in the reference image; and a defect determiner that determines whether the defect candidate region is a defect, on a basis of the direction of the edge in the inspection target image and the direction of the edge in the reference image at a position corresponding to the defect candidate region.

6. An inspection method comprising: acquiring an inspection target image, the inspection target image being a read image obtained by reading an inspection side of a recording material on which an output image is formed; extracting an edge from each of the inspection target image and a reference image to be used in inspecting the output image; extracting a defect candidate region having a possibility of a defect by comparing the inspection target image with the reference image; calculating a direction of the edge in the inspection target image and a direction of the edge in the reference image; and determining whether the defect candidate region is a defect, on a basis of the direction of the edge in the inspection target image and the direction of the edge in the reference image at a position corresponding to the defect candidate region.